Transversely-excited film bulk acoustic resonator with spiral interdigitated transducer fingers

ABSTRACT

Acoustic resonator devices, filers, and methods. An acoustic resonator includes a substrate and a piezoelectric plate, a portion of the piezoelectric plate being a diaphragm spanning a cavity in the substrate. A conductor pattern on a front surface of the piezoelectric plate includes interleaved interdigital transducer (IDT) fingers connected alternately to first and second busbars. The interleaved IDT fingers are on the diaphragm, and the interleaved IDT fingers include at least a first pair of interleaved spiral IDT fingers.

RELATED APPLICATION INFORMATION

This application is a continuation of application PCT/US21/58974, titled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH SPIRAL INTERDIGITATED TRANSDUCER FINGERS, filed Nov. 11, 2021, which claims priority from U.S. provisional application 63/107,950 titled “XBAR WITH SPIRAL TRANSDUCER” filed Oct. 30, 2020, the entire content of these applications is incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.

Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies and bandwidths proposed for future communications networks.

The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. Radio access technology for mobile telephone networks has been standardized by the 3GPP (3^(rd) Generation Partnership Project). Radio access technology for 5^(th) generation mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division duplexing (TDD), such that a communications device operating in band n77 and/or band n79 use the same frequencies for both uplink and downlink transmissions. Bandpass filters for bands n77 and n79 must be capable of handling the transmit power of the communications device. WiFi bands at 5 GHz and 6 GHz also require high frequency and wide bandwidth. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer, or diaphragm, of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers, extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm. XBAR resonators provide very high electromechanical coupling and high frequency capability. XBAR resonators may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are well suited for use in filters for communications bands with frequencies above 3 GHz. Matrix XBAR filters are also suited for frequencies between 1 GHz and 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectional views, and a detail view of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is a graphic illustrating a shear primary acoustic mode in an XBAR.

FIG. 3 is a schematic plan view of an interdigital transducer (IDT) electrode pattern with spiral fingers.

FIG. 4 is a schematic plan view of one spiral finger from the electrode pattern of FIG. 3.

FIG. 5A is a schematic plan view of an IDT electrode pattern with square spiral fingers.

FIG. 5B is a schematic plan view of one spiral finger from the electrode pattern of FIG. 5A.

FIG. 6 is a schematic plan view of another IDT electrode pattern with spiral fingers.

FIG. 7 is a schematic plan view of an IDT electrode pattern with spiral fingers and scalloped busbars.

FIG. 8 is a flow chart of a method for fabricating an XBAR.

Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view, orthogonal cross-sectional views, and a detailed cross-sectional view of a transversely-excited film bulk acoustic resonator (XBAR) 100. XBAR resonators such as the resonator 100 may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suited for use in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate 110 having parallel front and back surfaces 112, 114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. In the examples presented in this patent, the piezoelectric plates are Z-cut, which is to say the Z axis is normal to the front and back surfaces 112, 114. However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a surface of the substrate 120 except for a portion of the piezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140 formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm” 115 due to its physical resemblance to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of the piezoelectric plate 110 around all of a perimeter 145 of the cavity 140. In this context, “contiguous” means “continuously connected without any intervening item”. In other configurations, the diaphragm 115 may be contiguous with the piezoelectric plate around at least 50% of the perimeter 145 of the cavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate or may be attached to the substrate 120 via one or more intermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A and Section B-B) or a recess in the substrate 120 under the diaphragm 115. The cavity 140 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The direction parallel to the IDT fingers will be referred to herein as the “aperture direction”. The center-to-center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT. The direction perpendicular to the IDT fingers will be referred to herein as the “length direction.”

The first and second busbars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. As will be discussed in further detail, the primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate 110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the diaphragm 115 of the piezoelectric plate which spans, or is suspended over, the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may have more or fewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and mark (“mark” is a term commonly used to refer to the dimension perpendicular to the long axis of a conductor such as an IDT finger) of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR may have hundreds of parallel fingers in the IDT 110. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

Referring to the detailed cross-sectional view (Detail C), a front-side dielectric layer 150 may optionally be formed on the front side of the piezoelectric plate 110. The “front side” of the XBAR is, by definition, the surface facing away from the substrate. The front-side dielectric layer 150 may be formed only between the IDT fingers (e.g. IDT finger 138 b) or may be deposited as a blanket layer such that the dielectric layer is formed both between and over the IDT fingers (e.g. IDT finger 138 a). The front-side dielectric layer 150 may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. The thickness of the front side dielectric layer is typically less than or equal to the thickness of the piezoelectric plate. The front-side dielectric layer 150 may be formed of multiple layers of two or more materials. While the IDT fingers 138 a, 138 b are shown with rectangular cross-section shapes, other shapes are possible including trapezoidal.

The IDT fingers 138 a and 138 b may be aluminum, an aluminum alloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum or some other conductive material. The IDT fingers are considered to be “substantially aluminum” if they are formed from aluminum or an alloy comprising at least 50% aluminum. The IDT fingers are considered to be “substantially copper” if they are formed from copper or an alloy comprising at least 50% copper. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers and/or to improve power handling. The busbars 132, 134 of the IDT may be made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension m is the mark of the IDT fingers.

As shown in DETAIL C, IDT finger 138 a has a trapezoidal cross-sectional shape and IDT finger 138 b has a rectangular cross-sectional shape. The IDT fingers 138 a, 138 b may have some other cross-section, such as T-shaped or stepped. The IDT fingers 138 a, 138 b are shown as single layer structures which may be aluminum or some other metal. IDT fingers may include multiple layers of materials, which may be selected to have different acoustic loss and/or different acoustic impedance. When multiple material layers are used, the cross-sectional shapes of the layers may be different. Further, a thin adhesion layer of another material, such as titanium or chrome, may be formed between the IDT fingers 138 a, 138 b and the piezoelectric plate 110. Although not shown in FIG. 1, some or all IDT fingers may be disposed in grooves or slots extending partially or completely through the piezoelectric plate 110.

FIG. 2 is a graphical illustration of the primary acoustic mode of interest in an XBAR. FIG. 2 shows a small portion of an XBAR 200 including a piezoelectric plate 210 and three interleaved IDT fingers 230. A radio frequency (RF) voltage is applied to the interleaved fingers 230. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is primarily lateral, or parallel to the surface of the piezoelectric plate 210, as indicated by the arrows labeled “electric field”. Since the dielectric constant of the piezoelectric plate is significantly higher than the surrounding air, the electric field is highly concentrated in the plate relative to the air. The lateral electric field introduces shear deformation, and thus strongly excites a shear-mode acoustic mode, in the piezoelectric plate 210. Shear deformation is deformation in which parallel planes in a material remain parallel and maintain a constant distance while translating relative to each other. A “shear acoustic mode” is an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformations in the XBAR 200 are represented by the curves 260, with the adjacent small arrows providing a schematic indication of the direction and magnitude of atomic motion. The degree of atomic motion, as well as the thickness of the piezoelectric plate 210, have been greatly exaggerated for ease of visualization. While the atomic motions are predominantly lateral (i.e. horizontal as shown in FIG. 2), the direction of acoustic energy flow of the excited primary shear acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by the arrow 265.

When the piezoelectric plate 210 is Z-cut lithium niobate or lithium tantalate, the coupling between an electric field along either the X axis or the Y axis and the primary shear acoustic wave is essentially the same. In FIG. 2, the IDT fingers 230 may be perpendicular to the X crystalline axis, parallel to the X crystalline axis, or at an oblique angle to the X crystalline axis. In all cases, the electric field between IDT fingers excites a primary shear acoustic wave with atomic motion along the direction of the electric field and acoustic energy flow substantially orthogonal to the surface of the piezoelectric plate 210.

An XBAR based on shear acoustic wave resonances can achieve better performance than current state-of-the art surface acoustic wave (SAW), film-bulk-acoustic-resonators (FBAR), and solidly-mounted-resonator bulk-acoustic-wave (SMR BAW) devices. In particular, the piezoelectric coupling for shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. High piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters of various types with appreciable bandwidth.

FIG. 3 is a schematic plan view of an IDT 330 with interleaved spiral IDT fingers. The IDT 330 includes a first busbar 332, a second busbar 334, and fingers, such as fingers 336 and 338, extending from the busbars. Pairs of interleaved fingers, such as fingers 336 and 338, one of which extends from each of the first and second busbars, are curved into spiral shapes. The IDT 330 may be substituted, for example, for the IDT 130 in FIG. 1 such that the spiral fingers of the IDT 330 are disposed over the cavity (indicated by the dashed line 345) of an XBAR device. While FIG. 3 shows only two pairs of interleaved spiral fingers, the IDT of an XBAR may have more than two pairs. Further, while FIG. 3 shows only interleaved spiral fingers, and IDT may have a combination of spiral and straight interleaved fingers (i.e. as shown in FIG. 1). An IDT is considered to have spiral fingers if at least one pair of fingers are spirals as defined herein.

An IDT with interleaved spiral IDT fingers, such as the IDT 330, can have very long fingers without requiring a correspondingly wide diaphragm and cavity. An IDT with a reduced number of long fingers may have lower losses than an IDT with a greater number of shorter straight fingers. In particular, an IDT with spiral fingers may incur lower losses at the ends of the fingers.

The interleaved spiral fingers 336, 338 may have an effectively constant mark m and an effectively constant pitch p. In this context, the phrase “effectively constant” means “constant within a predetermined range.” The predetermined range may be selected such that variations in resonator resonance and anti-resonance frequencies due to variations in pitch and mark do not degrade the performance of a filter using the XBAR with spiral fingers. The range of pitch and mark that can be considered “effectively constant” will vary with specific filter designs. It is noted that the resonance and anti-resonance frequencies of XBARs are highly dependent on the thickness of the piezoelectric plate but are relatively insensitive to IDT pitch and mark. For example, changing the pitch of an XBAR from 7.5 times a thickness of the piezoelectric plate to 15 times the thickness of the piezoelectric plate changes the resonance frequency of the XBAR by about 3%, Further, as described, for example, in published patent application US20210067138A1, small variations in mark and/or pitch within an IDT may improve filter performance by suppressing spurious acoustic modes. Variations in pitch and mark are typically less than +1% but may be as high as ±5%.

The pitch p between the centers of adjacent conductors and the mark m are measured in a direction perpendicular to a tangent to the conductors at the point of measurement. When the two conductors are not precisely parallel at the point of measurement, the pitch p and the mark m are the average of two measurements made in directions perpendicular to tangents to each of the two conductors.

A rigorous definition of a spiral is “a curve on a plane that winds around a fixed center point at a continuously increasing or decreasing distance from the point.” However, in this patent, “spiral” is more broadly defined as “a curve on a plane that bends, continuously or in steps, through at least 180 degrees.” In particular, adjacent IDT fingers that bend at least 180 degrees while maintaining an effectively constant pitch are considered interleaved spiral IDT fingers. FIG. 4, which includes one busbar 334 and one finger 338 of the IDT 330, illustrates that the interleaved spiral IDT fingers of the IDT 330 curve a total of 720 degrees.

FIG. 5A is a plan view of another IDT 530 with interleaved spiral IDT fingers that bend in 90-degree steps. IDT fingers 536 and 538 extend from first and second busbars 532, 534, respectively. Fingers 536 and 538 each bend in 90-degree steps while maintaining a constant or effectively constant pitch p between adjacent conductor segments. The gap between adjacent conductors is a dimension g equal to the difference between the pitch p and the mark m. The distance between the first bus bar 532 and the upper (as shown in FIG. 5A) segment 538 a of finger 538 may be equal to g. Similarly, the distance between the second bus bar 534 and the lowest (as shown in FIG. 5A) segment 536 a of finger 536 may be equal to g.

FIG. 5B, which includes one busbar 534 and one finger 538 of the IDT 530, illustrates that the interleaved fingers of the IDT 330 bend a total of 630 degrees in seven 90-degree steps.

While the two fingers of a pair of interleaved spiral fingers must, by definition, bend at least 180 degrees, the above definition of interleaved spiral fingers does not preclude such fingers having straight sections. FIG. 6 is a plan view of another IDT 630 with interleaved spiral fingers. IDT fingers 636 and 638 extend from first and second busbars 632, 634, respectively. Each IDT finger has a single semicircular bend of 180 degrees, which is the minimum bend consistent with the previous definition of interleaved spiral fingers. Each finger includes straight segments. The pitch p between adjacent fingers is effectively constant for both the curved and straight segments of each finger.

FIG. 7 is a plan view of another IDT 730 with interleaved spiral fingers. IDT fingers 736 and 738 extend from first and second busbars 732, 734, respectively. Each IDT finger has two semicircular bends of 180 degrees, with a total bend of 360 degree. Each IDT finger includes straight segments. The pitch p between adjacent fingers is effectively constant for both the curved and straight segments of each finger.

Each busbar 732, 734 has concave recesses (772, 774 respectively) adjacent to the semicircular bends of the IDT fingers such that the gap between the bus bars and the adjacent curved IDT fingers is effectively constant. Specifically, the gap g between the first busbar 732 and the curved segment 738 a of IDT finger 738 is effectively constant and the same as the gap g between the straight segments of the IDT fingers. Similarly, the gap between the second busbar 734 and the curved segment 736 a of IDT finger 736 is effectively constant and the same as the gap g between the straight segments of the IDT fingers.

Description of Methods

FIG. 8 is a simplified flow chart summarizing a process 800 for fabricating a filter device incorporating XBARs with structures for reducing acoustic energy leakage. Specifically, the process 800 is for fabricating a filter device including multiple XBARs, some of which may include a frequency setting dielectric or coating layer. The process 800 starts at 805 with a device substrate and a thin plate of piezoelectric material disposed on a sacrificial substrate. The process 800 ends at 895 with a completed filter device. The flow chart of FIG. 8 includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 8.

While FIG. 8 generally describes a process for fabricating a single filter device, multiple filter devices may be fabricated simultaneously on a common wafer (consisting of a piezoelectric plate bonded to a substrate). In this case, each step of the process 800 may be performed concurrently on all of the filter devices on the wafer.

The flow chart of FIG. 8 captures three variations of the process 800 for making an XBAR which differ in when and how cavities are formed in the device substrate. The cavities may be formed at steps 810A, 810B, or 810C. Only one of these steps is performed in each of the three variations of the process 800.

The piezoelectric plate may typically be Z-cut lithium niobate. The piezoelectric plate may be some other material and/or some other cut. The device substrate may preferably be silicon. The device substrate may be some other material that allows formation of deep cavities by etching or other processing.

In one variation of the process 800, one or more cavities are formed in the device substrate at 810A, before the piezoelectric plate is bonded to the substrate at 815. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that two or more resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at 810A will not penetrate through the device substrate.

At 815, the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and the device substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the device substrate and the piezoelectric plate are highly polished. One or more layers of intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric plate and the device substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the device substrate or intermediate material layers.

At 820, the sacrificial substrate may be removed. For example, the piezoelectric plate and the sacrificial substrate may be a wafer of piezoelectric material that has been ion implanted to create defects in the crystal structure along a plane that defines a boundary between what will become the piezoelectric plate and the sacrificial substrate. At 820, the wafer may be split along the defect plane, for example by thermal shock, detaching the sacrificial substrate and leaving the piezoelectric plate bonded to the device substrate. The exposed surface of the piezoelectric plate may be polished or processed in some manner after the sacrificial substrate is detached.

A first conductor pattern, including IDTs of each XBAR, is formed at 830 by depositing and patterning one or more conductor layers on the front side of the piezoelectric plate. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. A second conductor pattern of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the first conductor pattern (for example the IDT bus bars and interconnections between the IDTs).

Each conductor pattern may be formed at 830 by depositing the conductor layer and, optionally, one or more other metal layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.

Alternatively, each conductor pattern may be formed at 830 using a lift-off process. Photoresist may be deposited over the piezoelectric plate. and patterned to define the conductor pattern. The conductor layer and, optionally, one or more other layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern.

At 850, one or more frequency setting dielectric layer(s) may optionally be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. For example, a dielectric layer may be formed over the shunt resonators to lower the frequencies of the shunt resonators relative to the frequencies of the series resonators. The one or more dielectric layers may be deposited using a conventional deposition technique such as physical vapor deposition, atomic layer deposition, chemical vapor deposition, or some other method. One or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate. For example, a mask may be used to limit a dielectric layer to cover only the shunt resonators.

At 855, a passivation/tuning dielectric layer is deposited over the piezoelectric plate and conductor patterns. The passivation/tuning dielectric layer may cover the entire surface of the filter except for pads for electrical connections to circuitry external to the filter. In some instantiations of the process 800, the passivation/tuning dielectric layer may be formed after the cavities in the device substrate are etched at either 810B or 810C.

In a second variation of the process 800, one or more cavities are formed in the back side of the device substrate at 810B. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that two or more resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities may be formed using an anisotropic or orientation-dependent dry or wet etch to open holes through the back side of the device substrate to the piezoelectric plate. In this case, the resulting resonator devices will have a cross-section as shown in FIG. 1.

In a third variation of the process 800, one or more cavities in the form of recesses in the device substrate may be formed at 810C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that two or more resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities formed at 810C will not penetrate through the device substrate.

Ideally, after the cavities are formed at 810B or 810C, most or all of the filter devices on a wafer will meet a set of performance requirements. However, normal process tolerances will result in variations in parameters such as the thicknesses of dielectric layer formed at 850 and 855, variations in the thickness and line widths of conductors and IDT fingers formed at 845, and variations in the thickness of the piezoelectric plate. These variations contribute to deviations of the filter device performance from the set of performance requirements.

To improve the yield of filter devices meeting the performance requirements, frequency tuning may be performed by selectively adjusting the thickness of the passivation/tuning layer deposited over the resonators at 855. The frequency of a filter device passband can be lowered by adding material to the passivation/tuning layer, and the frequency of the filter device passband can be increased by removing material to the passivation/tuning layer. Typically, the process 800 is biased to produce filter devices with passbands that are initially lower than a required frequency range but can be tuned to the desired frequency range by removing material from the surface of the passivation/tuning layer.

At 860, a probe card or other means may be used to make electrical connections with the filter to allow radio frequency (RF) tests and measurements of filter characteristics such as input-output transfer function. Typically, RF measurements are made on all, or a large portion, of the filter devices fabricated simultaneously on a common piezoelectric plate and substrate.

At 865, global frequency tuning may be performed by removing material from the surface of the passivation/tuning layer using a selective material removal tool such as, for example, a scanning ion mill as previously described. “Global” tuning is performed with a spatial resolution equal to or larger than an individual filter device. The objective of global tuning is to move the passband of each filter device towards a desired frequency range. The test results from 860 may be processed to generate a global contour map indicating the amount of material to be removed as a function of two-dimensional position on the wafer. The material is then removed in accordance with the contour map using the selective material removal tool.

At 870, local frequency tuning may be performed in addition to, or instead of, the global frequency tuning performed at 865. “Local” frequency tuning is performed with a spatial resolution smaller than an individual filter device. The test results from 860 may be processed to generate a map indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of a mask to restrict the size of the areas from which material is removed. For example, a first mask may be used to restrict tuning to only shunt resonators, and a second mask may be subsequently used to restrict tuning to only series resonators (or vice versa). This would allow independent tuning of the lower band edge (by tuning shunt resonators) and upper band edge (by tuning series resonators) of the filter devices.

After frequency tuning at 865 and/or 870, the filter device is completed at 875. Actions that may occur at 875 include forming bonding pads or solder bumps or other means for making connection between the device and external circuitry (if such pads were not formed at 845); excising individual filter devices from a wafer containing multiple filter devices; other packaging steps; and additional testing. After each filter device is completed, the process ends at 895.

Closing Comments

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

It is claimed:
 1. An acoustic resonator, comprising: a substrate; a piezoelectric plate, a portion of the piezoelectric plate being a diaphragm spanning a cavity in the substrate; and a conductor pattern on a front surface of the piezoelectric plate, the conductor pattern comprising interleaved interdigital transducer (IDT) fingers connected alternately to first and second busbars, wherein the interleaved IDT fingers are on the diaphragm, and the interleaved IDT fingers comprise at least a first pair of interleaved spiral IDT fingers.
 2. The acoustic resonator of claim 1, wherein each finger of the first pair of interleaved spiral IDT fingers bends at least 180 degrees.
 3. The acoustic resonator of claim 1, wherein a center-to-center spacing of the first pair of interleaved spiral IDT fingers is effectively constant.
 4. The acoustic resonator of claim 1, wherein a center-to-center spacing of the first pair of interleaved spiral IDT fingers is constant within a predetermined tolerance.
 5. The acoustic resonator of claim 4, wherein the predetermined tolerance is +5%.
 6. The acoustic resonator of claim 1, wherein each finger of the first pair of interleaved spiral IDT fingers comprises one or more semicircular curved portions.
 7. The acoustic resonator of claim 6, wherein each of the first and second busbars includes recesses such that gaps between each of the first and second busbars and curved portions of adjacent IDT fingers are effectively equal to a gap between adjacent interleaved IDT fingers.
 8. The acoustic resonator of claim 1, wherein each finger of the first pair of interleaved spiral IDT fingers comprises two or more 90-degree bends.
 9. The acoustic resonator of claim 8, wherein gaps between each of the first and second busbars and adjacent IDT fingers are effectively equal to a gap between adjacent interleaved IDT fingers.
 10. The acoustic resonator of claim 1, wherein the piezoelectric plate is one of Z-cut lithium niobate and Z-cut lithium tantalate.
 11. The acoustic resonator of claim 1, wherein all of the interleaved IDT fingers are interleaved spiral IDT fingers.
 12. A filter device, comprising: a piezoelectric plate, portions of the piezoelectric plate being diaphragms spanning respective cavities in a substrate; and a conductor pattern on a front surface of the piezoelectric plate, the conductor pattern comprising a plurality of interdigital transducers (IDTs), each IDT comprising interleaved IDT fingers connected alternately to first and second busbars, wherein the interleaved IDT fingers of each IDT are on a respective diaphragm, and the interleaved IDT fingers of a least one IDT comprise a pair of interleaved spiral IDT fingers.
 13. The filter device of claim 12, wherein each finger of the pair of interleaved spiral IDT fingers bends at least 180 degrees.
 14. The filter device of claim 12, wherein a center-to-center spacing of the pair of interleaved spiral IDT fingers is effectively constant.
 15. The filter device of claim 12, wherein the piezoelectric plate is one of Z-cut lithium niobate and Z-cut lithium tantalate.
 16. A method of fabricating an acoustic resonator, comprising: attaching a back surface of a piezoelectric plate to a substrate; forming a conductor pattern on a front surface of the piezoelectric plate, the conductor pattern comprising interleaved interdigital transducer (IDT) fingers connected alternately to first and second busbars; and forming a cavity in the substrate such that the interleaved IDT fingers are on a portion of the piezoelectric plate spanning the cavity, wherein the interleaved IDT fingers include at least a first pair of interleaved spiral IDT fingers.
 17. The method of claim 16, wherein each finger of the pair of interleaved spiral IDT fingers bends at least 180 degrees.
 18. The method of claim 16, wherein a center-to-center spacing of the pair of interleaved spiral IDT fingers is effectively constant.
 19. The method of claim 16, wherein the piezoelectric plate is one of Z-cut lithium niobate and Z-cut lithium tantalate. 